Function camel_to_snake_case 

pub fn camel_to_snake_case<I, O>(output: &mut O, input: I) -> Result
where
    I: Read,
    O: Write,

Streams ASCII camel-case bytes into a writer as lowercase snake case.

The underscore rule matches camel_to_snake_string. The first input read error stops processing and is not returned, while output formatting errors are propagated. Bytes outside ASCII are written as individual Unicode code points rather than decoded as UTF-8.